1 Nutrients •Katerina Dadakova, Department of Biochemistry •Figures adopted from Buchanan et al., Biochemistry & molecular biology of plants Nutrients Essential mineral nutrients Nutrients Potassium K+ is the most abundant cellular cation K+ functions: •osmoticum •charge balance •enzyme activation Nutrients Compound Oxidation state of N Name N2 0 Dinitrogen (nitrogen gas) HN3 -3 Ammonia NH4+ -3 Ammonium ion N2O +1 Nitrous oxide NO +2 Nitric oxide NO2- +3 Nitrite NO2 +4 Nitrogen dioxide NO3- +5 Nitrate Nitrogen nitrogen fixation glutamine synthetase N2 NH4+ Glutamine Organic nitrogen compounds Glutamate Selected organic nitrogen compounds Nitrogen deficiency phenotype Nutrients Plants may acquire N as: • ammonium ion •nitrate •dinitrogen, only in the case of plant species capable of endosymbiosis with nitrogen-fixing bacteria • Obtaining nitrogen through symbiosis consumes 12 to 17 g of carbohydrate per gram of N fixed Nutrients Nitrogen fixation N2 + 16ATP + 8e- + 8H+ 2NH3 + H2 + 16ADP + 16Pi Nitrogenase complex Dinitrogenase reductase Fe protein Dinitrogenase MoFe protein Nutrients An external file that holds a picture, illustration, etc. Object name is nihms171659f1.jpg Object name is nihms171659f1.jpg Legume-rhizobial symbiosis The plant creates root nodules to ensure: •microaerobic environment •organic acids to feed the bacteria •carbon skeletons to transport fixed nitrogen • Bacterial symbionts fix nitrogen and release the resulting ammonia • • 1.Plant signals 2.Nod factors 3.Nodulin proteins Nutrients Nodule morphogenensis In symbiosomes, bacteria differentiate into bacteroids Nutrients Microaerobic nodule environment is created Carbon is provided to the bacteroids as dicarboxylic acids •oxidation of DCA provides ATP •DCA carbon backbones are used for nitrogen transport Nutrients Ammonia assimilation GS-GOGAT cycle Nutrients Nitrate assimilation Nutrients Nitrate reductase NR homodimer NR reaction NO3- + NAD(P)H + H+ NO2- + NAD(P)+ + H2O FAD Heme -Fe MoCo Hinge II Hinge I NAD(P)H NAD(P)+ NO3- NO2- Nutrients Nitrite reductase NiR reaction NO2- + 6 Fdxred + 8 H+ NH4+ + 6 Fdxox + 2 H2O Nutrients Regulation of nitrate assimilation hours of nitrate exposure days Nutrients Sulfur APS – 5-adenylylsulfate PAPS – 3´phosphoadenosine-5´-phosphosulfate, Nutrients Phytochelatin molecule Nutrients Coenzyme A Thiamine pyrophosphate Nutrients Nutrients Sulfate activation Nutrients Sulfate reduction to sulfide Nutrients •The sulfate reduction in plants is still a matter of intensive research. •A family of cDNA isolated from Arabidopsis has been shown to encode plastid-localized enzymes with thiol-dependent APS reductase activity •The cDNAs are able to complement APS kinase and PAPS reductase mutants of E. coli, indicating that the enzyme uses APS as a substrate and that free sulfite is produced •The complementation depends on the ability of the E. coli strain to synthesize glutathione, supporting the idea that glutathione is the in vivo reductant in plants •The free sulfite reduction is catalyzed by sulfite reductase found in the plastids •Plant sulfite reductase is a homooligomeric protein, where each subunit contains one siroheme and one iron-sulfur cluster, just like the C-terminal half of nitrite reductase. •And also the sequence of the enzyme is homologous to the C-terminal region of NiR Cysteine synthesis Nutrients •Cysteine is synthesized from serine by the combined activities of serine acetyltransferase and O-acetylserine(thiol)lyase •First, O-acetylserine is formed from serine and acetyl-CoA •There are several sources of acetyl-CoA in plants, e.g. by the action of pyruvate dehydrogenase that generates acetyl-CoA and CO2 •OAS and the sufide ion then react to form cysteine •O-acetylserine(thiol)lyase contains pyridoxal phosphate as a prosthetic group Regulation of sulfate assimilation • Sulfur assimilation is not strongly regulated by light the enzymes are also active in etiolated plants and do not demonstrate diurnal oscillations • • Sulfur assimilation is regulated by developmental stage all the enzymes are highly active in young leaves and root tips • • Sulfur assimilation is regulated in response to the availability of sulfur sulfur starvation results in the up-regulation of sulfate transport and APS reductase • The content of reduced sulfur and nitrogen is strictly coordinated • Sulfite and sulfide are not allowed to accumulate Nutrients •The regulation is very different from regulation of nitrate or carbon assimilation •First, sulfur assimilation is not strongly regulated by light: although the activities of the sulfur assimilation enzymes increase severalfold when dark-grown plants are illuminated, these enzymes are also active in etiolated plants and do not demonstrate diurnal oscillations in activity (in contrast, nitrate reductase and Rubisco are not expressed in etiolated plants) •Sulfur assimilation is regulated by developmental stage: all the sulfur assimilation enzymes are highly active in young leaves and root tips and decline markedly in older tissues (in young actively growing tissues is actually a high demand for cysteine and methionine for protein synthesis) •Just as N-assimilating enzymes are regulated by nitrogen availability, sulfur assimilation is regulated in response to the availability of sulfur, but differently: sulfur starvation results in the up-regulation of sulfate transport and APS reductase. •What is the product of APS reductase-catalysed reaction? •Keeping the activity of APS reductase low, except of the case of sulfur starvation, makes sense, because the intermediates of this pathway, i.e. sulfite and sulfide, are toxic and must not be allowed to accumulate. •Sulfur assimilation is coordinated with the rate of nitrogen assimilation and with the growth rate so that the content of reduced sulfur and nitrogen in plants is strictly maintained at a ratio of 1:20 Phosphorus Nutrients Phosphate functions: •component of nucleic acids and phospholipids •energy conversion (ATP) •regulation Endomycorrhizae Root modifications in low Pi concentration •P can exist in plants as both inorganic phosphate anions and organophosphate compounds. Unlike nitrate and sulfate, phosphate is not reduced in plants during assimilation, but instead remains in its oxidized state, forming phosphate esters. Phosphate constitutes an important structural component of nucleic acids and phospholipids, plays a critical role in energy conversion, and is an important factor in regulation and signal transduction by way of protein phosphorylation. •Because of its low solubility, phosphate is relatively unavailable to plant roots. Hence, P supply is one of the major constraints of plant growth. Therefore, roots exhibit an impressive plasticity and can modify their structure to obtain soil P that would be otherwise unavailable. •A wide range of plant species create mycorrhizal associations to enhance the P absorption, the so-called endomycorrhizae. These soil fungi form extensive hyphal networks in the soil to acquire Pi. Within root cortical tissues, they form branched haustoria called arbuscules that exchange the solutes with the host plant. Arbuscules are in intimate associations with the plasma membrane but do not penetrate the cell. •Further, roots can release phosphatases to release organically bound P from the soil.